The Sheet Metal and Air Conditioning Contractors National Association (SMACNA) provides deflection criteria for certain proprietary flange systems; compliance is to be proved by testing, rather than by mathematical calculation. A flange design for which criteria have been established and recognized by the Association is known as the T-24. Competing manufacturers who have their own patented systems may appropriately test them as against the T-24 test results.
The T-24 flange consists of a first portion bent perpendicular to the sheet metal surface, a second portion bent back parallel to the surface, and a third portion bent reversedly outward and back against the outer surface of the second portion.
Proprietary flange designs patented within the last decade have added portions of sheet metal at or adjacent to the duct surface, rather than to the outstanding edge of the flange. Thus, according to U.S. Pat. No. 4,466,641 issued to The Lockformer Co., the flange structure (herein called the "TDC flange") includes a broad groove, indented from the duct surface immediately adjacent to the outstanding leg of the flange. In a competing flange design called the "TDF flange" and shown in U.S. Pat. No. 4,579,375 to Engel Industries, Inc., a bead is formed in the duct surface spaced forwardly of the outstanding leg of the flange.
According to structural engineering theory which has conventionally been accepted in designing sheet metal products, such additional material (provided by the groove in the first instance and by the bead in the second) would serve, along with a part of the flat sheet metal adjacent, as the bottom flange of a virtual "I" beam, adding to its combined bending resistance. Thus, these recently patented constructions suggest that if material is to be added to strengthen the integral flange arising from a sheet metal duct surface, it should be added at or adjacent to that surface.